1. Field of the Invention
The present invention relates to a radio base station in a cellular mobile communication system (hereinafter referred to simply as a radio base station), and more particularly to phase compensation for correcting a phase deviation caused in frequency conversion of signals received by antenna elements directed to each sector.
2. Description of the Related Art
Various mobile communication services including car/portable telephone service have been remarkably developed in the recent several years not only in Japan but also throughout the world. And in such mobile communication technology, amazing development is observed especially with regard to cellular car/portable telephone service.
A cellular mobile communication system covers its entire service area by a multiplicity of cells which correspond to service zones of individual radio base stations. In this system, there occurs little interference to any other than the relevant cell to which antenna elements are directed, and in order to eliminate any interference wave arriving from other mobile stations beyond the directions of antenna elements, each cell is divided into a plurality, e.g., six, of sectors, and a signal from any mobile station positioned in each sector is received by a plurality of antenna elements directed to the relevant sector.
The signal received by a plurality of antenna elements directed to each sector is low-noise amplified by an LNA (Low Noise Amplifier), and digital orthogonal I, Q signals of baseband are generated in a heterodyne receiver including an AGC circuit (hereinafter a circuit composed of nonlinear elements converting of the received signal into baseband signals will be referred to as a receiver). Subsequently the signals are processed in a beam former to be given an amplitude weight and a phase rotation, and then the signals are synthesized to be thereby formed into a desired sharp beam pattern to enhance the gain.
In this manner, if a multi-beam antenna or adaptive array antenna with digital signal processing is applied to a radio base station in a cellular mobile communication system, it becomes possible to attain enhancement of the gain by sharpening the beam pattern equivalently and also to increase relatively easily the number of users accommodatable in a single cell by the action of reducing the intra-area interference.
However, relative to adoption for a radio base station in a cellular mobile system, the known beam forming technique with the conventional digital signal processing is still in a stage of research, and in most cases the study is carried out with computer simulations on the supposition of an ideal environment (where no phase deviation is existent between the receiver outputs (hereinafter referred to as antenna branches) derived from the input signal to antenna elements).
In the case of employing a multi-beam antenna, beam forming from the outputs of a plurality of antenna branches is executed on the supposition that there is no phase deviation between the antenna branches, so that when any phase deviation is existent between the antenna branches due to manufacture errors, secular changes, temperature characteristics and so forth in nonlinear elements such as LNAs, and mixers, a desired beam pattern fails to be attained to consequently bring about deterioration of the characteristics.
Meanwhile in the case of employing an adaptive array antenna, it may be considered that beam forming is executed without such problem despite any phase deviation between the antenna branches, since the amplitude and the phase are controlled inclusive of the phase deviation. In a transmission mode where beam forming is executed on the basis of controlled variables in reception in adaptive processing, separation of such controlled variables of the amplitude and the phase from the phase deviation is a requisite condition. However, it has been impossible in the prior art heretofore to realize such separation of the controlled variables.